This invention relates to a method and apparatus for forming and depositing a dielectric layer on a substrate. More particularly, it relates to using supercritical fluids as mediums in which caged-siloxane precursors are linked and deposited to form dielectric films with dielectric constants below 3.0 for use in integrated circuit fabrication.
Supercritical fluids are well defined in the art. Supercritical fluids or solutions exist when the temperature and pressure of a solution are above its critical temperature and pressure. In this state, there is no differentiation between the liquid and gas phases and the fluid is referred to as a dense gas in which the saturated vapor and saturated liquid states are identical. Near supercritical fluids or solutions exist when the reduced temperature and pressure of a solution are both greater than 0.8 but the solution is not yet in the supercritical phase. Due to their high density, supercritical and near supercritical fluids possess superior solvating properties.
Supercritical fluids have been used in thin film processing and other applications as developer reagents or extraction solvents. Morita et al. (U.S. Pat. Nos. 5,185,296 and 5,304,515) describe a method in which supercritical fluids are used to remove unwanted organic solvents and impurities from thin films deposited on substrates. Allen et al. (U.S. Pat. No. 5,665,527) describe a high resolution lithographic method in which a supercritical fluid is used to selectively dissolve a soluble unexposed portion of polymeric material from a substrate, thereby forming a patterned image. In recognition of the superior solvating properties of supercritical fluids, Steckle et al. (U.S. Pat. No. 5,710,187) describe a method for removing impurities from highly cross-linked nanoporous organic polymers. A key function of the supercritical solvent in the Steckle et al. method is to penetrate the nanoporous structure effectively so as to remove all traces of organic solvents and unreacted monomer.
Methods for depositing thin films using supercritical fluids also have been reported. Murthy et al. (U.S. Pat. No. 4,737,384) describe a method for depositing metals and polymers onto substrates using supercritical fluids as the solvent medium. Sievers et al. (U.S. Pat. No. 4,970,093) teach a chemical vapor deposition method (CVD), in which a supercritical fluid is used to dissolve and deliver a precursor in aerosol form to a conventional CVD reactor. Watkins et al. (U.S. Pat. No. 5,789,027) describe a method termed Chemical Fluid Deposition (CFD) for depositing a material onto a substrate surface. In this method a supercritical fluid is used to dissolve a precursor of the material to be deposited. This is done in the presence of the substrate. Once dissolved, a reaction reagent is introduced that initiates a chemical reaction involving the precursor, thereby depositing the material onto the substrate. This method takes advantage of supercritical fluids as mediums for reagent transport, reaction, and removal of impurities.
Although the described methods take advantage of the unique properties of supercritical fluids, the utility of supercritical fluids in semiconductor fabrication has only begun to be realized.
Dielectric films are of great importance in the microelectronics industry. Modern integrated circuit design relies heavily on the deposition of high-purity dielectric films on substrates. With integrated circuit designs and line-widths becoming smaller and smaller, the need for dielectrics with lower dielectric constants (low-k materials) is more critical due to capacitance effects set up between conducting and non-conducting layers.
Low-k materials are now commonly made from organic or inorganic polymers as reported by Hacker, Materials Research Society Bulletin, vol. 22, no. 10, pp. 33-38. Low-k organic polymers generally do not possess the thermal or mechanical properties required for integrated circuit applications, i.e. they have low glass transition temperatures, are too soft, or have poor mechanical properties. Inorganic polymers are often based on siloxane polymerization chemistry, in which the resultant polymers closely mimic silicon dioxide structure. The term siloxane is used in this application to refer to any class of molecules having at least one organic group and oxygen(s) bound to silicon. Silsesquioxanes are a class of siloxanes having the empirical formula RSiO1.5 where R is any organic group. Silsesquioxanes are common precursors for forming polymeric silicon-dioxide-like dielectric materials. Silsesquioxanes can have essentially two-dimensional ladder-type or well defined three-dimensional caged structures. Upon polymerization (typically done with a high temperature bake) both structure types form ladder-type cross-linked polymers. In the latter scenario, the cage structures are destroyed in the baking process in favor of a new cross-linked matrix. Although these inorganic polymers have the required thermal properties and toughness for integrated circuit applications they are often brittle and the high temperature curing raises the dielectric constant of the final material relative to pre-bake material. Incorporation of organic moieties into the inorganic polymer matrix can add some resistance to cracking relative to purely inorganic matrices as well as lower dielectric constants, but these issues remain problematic.
Nanoporous silica can provide low-k dielectric materials as reported by Changming et al., Materials Research Society Bulletin, vol. 22, no. 10, pp. 39-42. In this case the major variable determining the dielectric constant is the density of the material. As the density decreases the dielectric constant decreases, and thus porosity is important. Nanoporous silica films are commonly formed by depositing a monomeric precursor such as tetraethyl orthosilicate or TEOS (a siloxane) onto a substrate using a solvent, and then cross-linking the precursors to form a continuous porous solid network. The resultant films are dried by direct solvent evaporation or treatment with supercritical fluids. The films are then subjected to a high-temperature annealing step. Such films suffer from mechanical instability and cracking due to thermal expansion mismatch stresses during the high temperature annealing.
Miller et al., Materials Research Society Bulletin, vol. 22, no. 10, pp. 44-48, report the development of inorganic-organic nanophase-separated hybrid polymer materials. These materials comprise organic polymers cast with silsesquioxane-based ladder-type polymeric structures. A casting solvent is used to dissolve the inorganic and organic polymer components. The materials are spin coated onto a substrate and upon application of high temperature, the hybrid phase-separated polymer materials are formed. The materials may offer some improvements in crack resistance over bulk-fused silica. The high process temperatures associated with this method can be a liability to achieving desirable low-k materials.
In spite of the considerable research effort that has gone into developing new low-k dielectric materials and methods of integrating them into conventional IC fabrication procedures, there is still considerable room for improvement both in selection of the materials and development of better deposition processes.
Generally, the invention provides methods and apparatus for forming thin films possessing low dielectric constants (e.g., dielectric constants below 3.0) on integrated circuits or other substrates. The method of the invention features linking caged-siloxane precursors in such a way as to form dielectric layers which exhibit low dielectric constants by virtue of their silicon dioxide-like molecular structure and porous nature. Supercritical fluids may be used as the reaction medium and developer both to the dissolve and deliver the caged-siloxane precursors and to remove reagents and byproducts from the reaction chamber and resultant porous film created. Since the deposition of the thin film dielectric occurs under supercritical or near supercritical fluid conditions (e.g., about 50xc2x0 C. and 1000 psi) and does not require a subsequent high-temperature anneal, the method offers advantages over conventional methods. In addition, the apparatus of the invention features a way to selectively protect one side of the integrated circuit or other substrate under the high-pressure conditions necessary for supercritical fluid formation and use.
One aspect of the invention pertains to integrated circuits or partially fabricated integrated circuits including a dielectric layer, which has multiple chemically linked caged siloxane moieties. In one example, the caged siloxane moieties are silsesquioxanes. In a particularly preferred embodiment, the caged siloxane moieties are obtained from polyhedral oligomeric silsesquioxanes (POSS) monomers. In order to introduce voids into the dielectric layer, the caged siloxane moieties preferably have a cage diameter of between about 5 and 50 Angstroms, and more preferably between about 10 and 20 Angstroms.
To provide a stable linkage, the caged siloxane moieties employ various side groups such as organic linkers. In one embodiment, a polymerization reaction links the caged siloxane moieties to one another. Examples of suitable polymerizable side chains include acrylates, methacrylates, carboxylic acids, carboxylic acid halides, halosilanes, carboxylic acid esters, sulphonic acid esters, carbamoyl halides, epoxides, isocyanates, nitrites, olefins, styrenes, amines, alcohols, alkyl halides, aryl halides, sulphonic acids, sulphonic acid halides, phosphines, silanols, and silanes. In another example, the caged siloxane moieties are linked to one another by non-covalent self-assembly. In an alternative embodiment, the caged siloxane moieties are grafted on the integrated circuit or partially fabricated integrated circuit. They may be grafted using the reactive side chains identified above, for example. Regardless of the linking or bonding technique, the caged siloxane moieties in the dielectric layer may be either homogeneous or mixed (containing at least two chemically distinct species of caged siloxane).
The dielectric layers should possess certain physical properties that impart both mechanical strength and a low dielectric constant. To provide a low dielectric constant, the dielectric layers should have a significant amount of void space. In one embodiment, the dielectric layer has a porosity of between about 25% and 75%. More preferably, the dielectric layer has a porosity of between about 40% and 60%. Preferably, the dielectric layer has an average pore size of at most about 200 Angstroms; more preferably between about 5 and 25 Angstroms. Preferably, the dielectric layer has a dielectric constant of at most about 3.5, more preferably at most about 2.5, and most preferably at most about 2.
The dielectric layers of this invention may be used as inter-layer dielectrics, which electrically separate the substrate from a first metal layer in integrated circuits. Or they may be used as inter-metal dielectrics, which electrically separate two or adjacent metal layers. Preferably, the dielectric layers are used in a damascene process. In this context, they may be used as either the first or second layer of a dual damascene dielectric structure. In a typical integrated circuit embodiment, the dielectric layer has a thickness of at most about 10,000 Angstroms. For some applications, the thickness will be at most about 5,000 Angstroms.
Another specific aspect of this invention pertains to methods of forming a dielectric layer in an integrated circuit. The method may be characterized by the following sequence: (a) contacting a partially fabricated integrated circuit with a carrier containing caged siloxane species; and (b) linking the caged siloxane species to form the dielectric layer on the partially fabricated integrated circuit. As suggested above, the carrier is preferably a supercritical fluid or near-supercritical fluid. Examples of suitable supercritical fluids for use with this invention include supercritical carbon dioxide, supercritical ammonia, supercritical water, supercritical ethanol, supercritical ethane, supercritical propane, supercritical butane, supercritical pentane, supercritical dimethyl ether, supercritical hexafluoroethane, and mixtures thereof.
The process may involve converting the carrier to a supercritical state, either prior to or upon contact with the partially fabricated integrated circuit. Typically the carrier is removed (e.g., flushed from the partially fabricated integrated circuit) after linking the caged siloxane species. The carrier may also be used to remove byproducts of the linking reaction and/or unreacted caged siloxane species.
Preferably, the caged siloxane species used in the process contain the caged siloxane moieties, linking moieties, and other chemical features as described above. For example, the caged siloxane species may be of tris(dimethylvinyl)-POSS, methacrylfluoro(3)-POSS, or methacrylfluoro(13)-POSS. These or other caged siloxane species may be linked to one another by polymerization or non-covalent self assembly for example.
After the dielectric layer is formed in accordance with this invention, it may be further processed as required by any integrated circuit fabrication procedure. For example, the layer may be etched the dielectric layer to forms at least one of vias and trenches. Preferably, a damascene process is employed.
Yet another aspect of this invention pertains to apparatus allowing formation of a low dielectric constant dielectric layer on a partially fabricated integrated circuit. The apparatus may be characterized by the following features: (a) a reaction vessel capable of withstanding pressure and temperature associated with generating and/or using a supercritical or near-supercritical fluid; and (b) a chuck for positioning and holding the partially fabricated integrated circuit to allow depositing a dielectric layer on an active side of the partially fabricated integrated circuit, while protecting a back side of the partially fabricated integrated circuit. The apparatus may also include a source of the supercritical or near-supercritical fluid. It may also include a source of caged siloxane species.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated figures.